There are several types of 3D fabrication. These include but are not limited to fused deposition modeling that is normally seen in the personal consumer market using PLA or ABS plastic filament. In this method layer by layer plastic deposition is used to build a construct. In personal consumer versions 1.75 mm or 3 mm plastic filaments are run through a printing mechanism that heats and deposit the plastic in thin layers. Cheap consumer versions already allow for fine resolutions ranging from 50 um to 400 um. There are additional 3D fabrication methods such as selective laser sintering that fuse metal powders at a much finer resolution in layers, and injection molding which entails the injection of molten fabrication material into a mold, then rapid cooling of the material to create the desired device. Many of these methods are described in detail in “A Review of Additive Manufacturing,” by Wong and Hernandez in ISRN Mechanical Engineering 2012 Article ID 208760. More specific examples of injection molding can be found in “A review of micro-powder injection moulding as a microfabrication technique” and “Recent Methods for Optimization of Plastic Injection Molding Process—A Retrospective and Literature Review” found in Journal of Micromechanics and Microengineering Article ID 043001 and International Journal of Engineering and Science and Technology Volume 2, 2010, respectively. Also incorporated by reference is the article Weisman, Jeffery A., et al. “antibiotic and chemotherapeutic enhanced three-dimensional printer filaments and constructs for biomedical applications.” International Journal of Nanomedicine 10 (2015): 357 as well as the doctoral dissertation of Jeffery Adam Weisman Nanotechnology and additive manufacturing platforms for clinical medicine: An Investigation Of 3D Printing Bioactive Constructs And Halloysite Nanotubes For Drug Delivery And Biomaterials by Weisman, Jeffery A., Ph.D. Louisiana Tech University. 2014: 287 pages; 3662483.
3D printing by fused deposition modeling requires a plastic filament. A commercial extrusion device can normally make this filament. Normally plastic pellets of the desired material are run through the extrusion machine to create a filament. These pellets are normally the same or similar to those used in injection molding. The high costs of filament combined with the low cost of injection molding pellets has led to the recent creation of personal filament extrusion devices. The Lyman filament extruder was one of the first general personal designs to be built by the ends user or DIY for a low cost. This then lead to the sale of cheaper consumer oriented extrusion devices. One of the first personal filament extruders is Extrusionbot, LLC out of Phoenix Ariz. Custom 3D print filaments have been created with unique properties for circuit design as seen in “A Simple, Low-Cost Conductive Composite Material for 3D Printing of Electronic Sensors” by Simon Leigh 2012 DOI: 10.1371/journal.pone.0049365.
The general operation of filament extrusion devices is relatively simple. Pellets are poured into a hopper. They pass into a chamber or pipe with a moving auger in side. The pellets are moved down the pipe by auger. The chamber is heated by a heating mechanism to cause the pellets to melt and a melt-flow to occur. The heat level can be customized to the desired temperature. The end of the chamber or pipe will have a die with a hole drilled in with the diameter of the desired filament. As the molten plastic exits the die it will rapidly harden creating a filament. For certain materials, extra cooling measures must be taken, however this is not often seen with PLA or ABS (common 3D printing materials).
Most consumer filament extruders and printers use PLA or ABS plastic. Although there are more novel filaments that are for sale made from pellets such as nylon or a saw dust/plastic mix called laywood. This allows for fabrication of very unique filaments for unique constructs. To color PLA or ABS plastic pellets, a coloring powder is added into the hopper of the extruder. This colorant is normally not uniformly distributed but this is not usually visible to the naked eye.
Plastic melting point or meltflow temperatures are an intrinsic property of the material, and can be provided by the manufacturer. To enable ease of extrusion of the material, the heat applied to the extruding material must approach this point, but not exceed as a full melt of the plastic is not desired. Should a full melt be achieved, the material will not cool rapidly enough upon exit from the device to achieve a uniform diameter desired by the user. It has long been known that there are many variables in determining melt flow temperatures and material handling as seen in “Polymer Melt Flow Instabilities in Extrusion: Investigation of the Mechanism and Material and Geometric Variables” by Ballenger Trans. Soc. Rheol. 15, 195 (1971) and “The Case for Polylactic Acid as a Commodity Packaging Plastic” by Sinclair DOI:10.1080/10601329608010880.
Filament extruders need to be cleaned before differing batches of filament are extruded. This cleaning process can be difficult as plastics and additives can adhere to both the pipe and auger. Purging the extruder between batches takes substantial amounts of time. In medical situations requiring different plastics this could cause time delays between unique extrusions. Additionally, the need for sterilization would require the entire extrusion machine to be disassembled.
In the context of sterilization, it should be noted that an extruder for filaments is normally run from 160-220 Celsius depending on the plastic used, and that a 3D printer head normally runs from 200-230 Celsius depending upon the material and the surrounding environmental conditions. These temperatures are highly variable depending on the material used and the environmental conditions in which the materials are being printed. For example, Polycaprolactone (PCL) plastics melt at 60 Celsius and have been printed at 160 Celsius, however this still is not normally significant sterilization for many medical applications. This can be seen in the published application “Use of polycaprolactone plasticizers in poly(vinyl chloride) compounds,” US 20140116749 A1.
There have been multiple instances in the medical profession of quick fabrication of proto-type medical devices in practical and emergency situations. Practical applications where this is seen include the use of rapidly curing mixtures of poly-methyl methacrylate powder and liquid methyl-methacrylate (a known cytotoxic material and carcinogenic) for use in implantation of devices such as antibiotic loaded beads or as cushioning material for hip replacements. A plastic trachea for an infant was recently printed to be used as an emergency airway until a more stable implant could be devised. “Treatment of severe porcine tracheomalacia with a 3-dimensionally printed, bioresorbable, external airway splint” David A. Zopf; Colleen L. Flanagan; Matthew Wheeler; Scott J. Hollister; Glenn E. Green JAMA Otolaryngology—Head and Neck Surgery. 2014; 140(1):66-71.
Implanting standard plastics can be dangerous since bacteria easily adhere to them. This is problem in both medical and food processing. It can be seen in PVC endotracheal tubes as shown in Biomaterials. 2004 May; 25(11):2139-51 “Inhibition of bacterial adhesion on PVC endotracheal tubes by RF-oxygen glow discharge, sodium hydroxide and silver nitrate treatments.” This can also be seen in Maple Syrup digest October 1985 “Bacterial Adhesion to plastic tubing walls” by Warren King. The current level of medical printing technology would benefit from the ability to affordably add bioactive elements to devices or use non-toxic plastics to overcome potential implantation infections or inherent implant toxicity that may occur.
One issue with plastics that do not degrade such as PMMA involves the need for the later surgical removal of antibiotic beads when delivering antibiotics. Additional information on PMMA biomaterials can be found within US patents application and the references they incorporate, numbered but not limited to: application Ser. No. 13/446,775 Filed: Apr. 13, 2012 Title: Ceramic Nanotube Composites with Sustained Drug Release Capability for Implants, Bone Repair and Regeneration.
The literature shows a clear need for better designed medical related 3D printing methods and materials. In particular, methods and equipment to create bioactive or drug eluting constructs.